The present invention relates generally to a rain sensor for detecting rain conditions on the windshield of an automotive vehicle so that the drive speed of the windshield wipers can be adjusted according to the rain conditions. More specifically, the invention relates to a rain sensor in an automotive wiper speed control system which controls in particular the drive interval in the intermittent mode of a wiper drive system in accordance with rain condition.
A co-pending U.S. patent application, Ser. No. 527,776 filed on Aug. 30, 1983, discloses a Wiper Speed Control System for an Automotive Windshield Wiper, and the same invention has, in turn, been disclosed in the corresponding European patent application No. 83108597.2, filed Aug. 31, 1983. The disclosed windshield wiper speed control system controls the wiper driving speed according to rain conditions around the front windshield. The wiper speed control system includes a rain sensor for detecting rain conditions such as how heavily rain is falling and/or the average size of the individual raindrops. In the present disclosure, including the appended claims, the word "rain condition" is intended to include the meaning of the impact speed of rain drops onto the windshield and/or the rain sensor depending upon vehicle speed.
The rain sensor is positioned near the front windshield, for example, on the cowl top, the engine hood or the like, where rain falls in approximately the same manner as it does at the front windshield. The rain sensor is adapted to detect vibrations due to the impact of rain drops onto the sensor to produce a rain condition signal indicative of the detected rain conditions.
The automotive vehicle has various vibration sources causing vibrations in the vehicle body, such as the engine which creates vibrations as it runs and the suspension mechanism which tends to transmit vibrations from the road wheels to the vehicle body. Due to such vehicle body vibrations, the rain sensor may be spuriously vibrated or oscillated to produce noise in the rain condition signal which may obscure the rain condition information. Therefore, it is essential to provide a noise-free rain sensor which can satisfactorily and successfully avoid the influences of vehicle body vibrations.